Most of the superabsorption polymers used today for their ability to take up large amounts of liquid (water, urine) in a short time are primarily lightly crosslinked synthetic polymers. They include, for example, polymers and copolymers based on acrylic acid or acrylamide, which are not based on renewable raw materials and which are insufficiently biodegradable, if at all.
Superabsorbent polymers were initially developed with the focus solely on a very high swellability on contact with liquid, known as absorption or free swelling capacity (FSC), but it was subsequently determined that it is not just the amount of liquid which is absorbed that is important but also the gel strength. Absorption capacity on one hand and gel strength of a crosslinked polymer on the other hand, however, are contrary properties, as is already disclosed in U.S. Pat. No. 3,247,171 and U.S. Pat. Re 32,649. Superabsorbent polymers having a particularly high absorption capacity have little strength in the swollen gel state, so that a confining pressure, for example pressure due to the body of the wearer of a hygiene article, will cause the gel to deform and block further liquid distribution and absorption. According to U.S. Pat. Re 32,649, a balance should therefore be sought between the absorption capacity and the gel strength in order that, when such superabsorbent polymers are used in a diaper structure, they ensure liquid absorption, liquid transport, diaper dryness and skin dryness.
A factor in this connection is not just that the freely swollen superabsorbent polymer be able to retain the absorbed liquid under a subsequent application of a pressure, but also that the superabsorbent polymer be capable of absorbing liquids even against a simultaneously (i.e. during the liquid absorption process) exerted pressure of the kind encountered in practice when an infant or adult sits or lies on a sanitary article or when shearing forces are developed, for example as a result of motion of the legs. This specific absorption characteristic is referred to in Edana method 442.1-99 as “Absorbency Against Pressure” or AAP for short. The AAP value reported for a superabsorbent polymer is based on the pressure employed, for example, 21 g/cm2 at 0.3 psi and 50 g/cm2 at 0.7 psi. Also, the AAP may be based on the ratio chosen for the measurement of the superabsorbent polymer weight to area, for example 0.032 g per cm2, and also by the particle size distribution of a granular superabsorbent polymer.
Patents EP 0 538 904 B1 and U.S. Pat. No. 5,247,072 disclose superabsorbent polymers based on carboxyalkyl polysaccharides. To turn the carboxyalkyl polysaccharide into a superabsorbent polymer, the carboxyalkyl polysaccharide is dissolved in water, isolated by drying or precipitation and subsequently thermally crosslinked via internal ester bridges formed by the reaction of the hydroxyl groups of the polysaccharide skeleton with the acidic carboxyl groups. Since this crosslinking reaction is very sensitive to small changes in the pH, temperature or reaction time, the superabsorbent polymers obtained have fluctuating absorption properties. The materials are notable for a high absorbency under load value which, however, deteriorates to a fraction of the initial value after ageing for a few weeks.
U.S. Pat. No. 5,550,189 discloses superabsorbent polymers based on carboxyalkyl polysaccharides that possess improved ageing stability owing to the addition of at least two-functional crosslinkers such as for example aluminum salts or citric acid. The superabsorbent polymers are prepared from a conjoint homogeneous aqueous solution of carboxyalkyl polysaccharide and crosslinkers, in which solution the components are present in low concentration and from which they are conjointly isolated and then thermally crosslinked. The synthesis of these superabsorbent polymers is very energy and time intensive, since the aqueous solutions are very weak. The improved ageing stability as it is reported in the majority of the illustrative embodiments does not meet actual service requirements.
EP 855 405 A1 addresses the poor ageing stability of the absorption capacity of swellable starch maleates and proposes by way of solution to this problem adding mercapto compounds to the double bond of the maleic acid substituent. The absorption performance of the product, especially under a confining pressure, is very poor.
U.S. Pat. No. 4,952,550 describes a method of making a superabsorbent polymer based on carboxymethylcellulose by treating the carboxymethylcellulose in water or organic solvents with polyvalent metal salts and a hydrophobicity agent. There is no thermal crosslinking step. According to the disclosure, the gel blocking of these superabsorbent polymers is reduced by the hydrophobicity agent.
The raw materials for preparing polysaccharide based superabsorbent polymers are frequently soluble in water and have to be converted into a water-insoluble form for use as superabsorbent polymers for hygiene applications. Numerous existing processes involve a homogeneous crosslinking for the absorbent material in order that the water solubility of the absorbent may be reduced. This frequently has the disadvantage that such homogeneously crosslinked superabsorbent polymers no longer have the desired absorption capacity for liquids, since the swellability is excessively constrained by the crosslinking of the polymer chains.
Furthermore, homogeneous crosslinking compromises the biodegradability of the superabsorbent polymer, since the constrained swelling reduces the access for micro-organisms. In addition, the additionally introduced substituents inhibit enzymatic degradation [Mehltretter et al., Journal of the American Oil Chemists Society, 47 (1970) pages 522-524]. Attempts to ameliorate these disadvantageous properties have led to various surface treatment proposals.
U.S. Pat. No. 5,811,531 discloses the preparation of a superabsorbent polymer on the basis of polysaccharides, such as xanthan, which contain uronic acid groups by reacting the polysaccharides at the surface with at least two-functional organic crosslinkers. According to the disclosure, the products possess better free-swell absorbing ability against salt solutions than carboxyalkylated polysaccharides where the carboxyl groups are not attached directly to the saccharide units but via alkyl groups.
U.S. Pat. No. 5,470,964 discloses a process for preparing a superabsorbent polymer providing improved absorbency under load that is based on polysaccharides containing acid groups and is surface crosslinked by polyvalent metal ions. The disadvantages of this process are that the improved absorbency under load is achieved by the crosslinking of a relatively thick surface layer and that, according to the disclosure, this is only possible through prior incipient swelling of the polysaccharide with a large amount of solvent. The incipiently swollen state then allows sufficiently deep penetration of the polyvalent metal ions into the surface. To achieve this, the polysaccharide is introduced into an excess of the aqueous metal salt solution such that the weight ratio of polysaccharide to water is from 1:2 to 1:40. The thick crosslinked surface layer does provide good absorbency under load values, but the free swell capacity and also the retention capacity of the absorbent are disadvantageously reduced as a result. The process described has the further disadvantage that the polysaccharide portion added last to the crosslinker solution in the course of the manufacturing operation has less time to swell and encounters a lower crosslinker concentration, resulting in an inhomogeneous distribution of the crosslinker on the surface and hence fluctuations in the absorption properties.
U.S. Pat. No. 4,043,952 discloses the surface treatment of water-swellable anionic polyelectrolytes with polyvalent metal ions in a dispersing medium in which the polymer is insoluble to improve the dispersibility of the water-absorbent products.
The broad object underlying the invention is to overcome the disadvantages arising from the state of the art.
It is an object of the present invention to provide biodegradable superabsorbent polymers based on renewable raw materials that are free of the defects described above. More particularly, the superabsorbent polymers shall have very long term storage stability with very substantial retention of the absorption properties. The absorbent particles shall also possess high mechanical robustness in order that the formation of fines in the course of processing operations such as, for example, screening or conveying may be avoided. Furthermore, with regard to the absorption performance, the superabsorbent polymers shall not gel-block and shall possess not only a high absorption and retention capacity but also a high absorbency against pressure with regard to water and aqueous solutions. Moreover, for an effective absorption and in-use performance, the superabsorbent polymers shall have an overwhelmingly insoluble character even in an excess of aqueous solution.
It is a further object of the present invention to provide a process for preparing such superabsorbent polymers which is simple, economical and safe to carry out, which provides consistent product quality and which utilizes little solvent and ideally no organic solvent. Moreover, the processes shall not require toxicologically suspect substances to carry out.
A further object according to the invention consists in improving the biodegradability of hygiene articles such as sanitary napkins, wound dressings, incontinence articles and diapers.